Travelling light

EU research programme leads to development of modular carbon fibre tank claimed to hold more liquid hydrogen at greater pressure. Siobhan Wagner reports.


Future liquid hydrogen-powered vehicles could travel over 300 miles before re-filling, thanks to the development of a modular carbon fibre tank.

Designed by BMW’s R&D arm Forschung und Technik and a team of 34 partners from European aerospace automotive and supply industries, plus universities and research institutes, the developers say the tank could be ready for commercial use in five years.

It is the result of an EU-funded research programme StorHy, which aims to improve current high-pressure, liquid and solid hydrogen storage technologies.

Using carbon fibre meant it was possible to reduce the weight of the entire tank storage system by a third compared with current cylindrical steel tanks used in hydrogen cars such as the BMW Hydrogen 7. The tank’s modular design is also claimed to be less cumbersome than cylindrical steel tanks, which take up most of a car’s boot space.

The tank, which is in the shape of a rectangular block, is made up of a series of layers. The inner structure consists of tank modules that connect like Lego blocks. ‘It can be scaled to any size,’ said Daniel Kammerer, head of BMW’s CleanEnergy communications division.

The connected modules add strength to the tank in the same way that bulkhead designs add strength to a submarine. But while submarines are designed to resist external pressure, the tank is designed to hold greater internal pressure. The more pressure it can hold, the more hydrogen it can store.

Kammerer claims the tank holds more hydrogen than a cylindrical tank.

‘The steel tank could only hold 8kg safely, whereas the new carbon fibre tank can hold up to 10kg,’ he said.

While many other car manufacturers are looking to bring hydrogen-fuel cars to the market, most have settled on using hydrogen in its compressed gaseous form.

Kammerer said BMW uses liquid hydrogen because it is denser than hydrogen gas even when compressed to 690 bar.

The main challenge is getting hydrogen into liquid form — which can only be achieved by chilling it to -253ºC.

The extremely low temperature requires lots of insulation, which unfortunately makes storage tanks bulky. Yet even with all this insulation, liquid hydrogen still warms up over time and boils. Afterwards the vapour hydrogen is vented into the atmosphere until the tank empties.

Kammerer said it takes longer for hydrogen to boil off in the modular carbon fibre tank. ‘With the former steel tank, hydrogen would start to boil off after 17 hours,’ he said. ‘The boil-off time is now three days,’ he claimed.

BMW’s hydrogen-powered cars run on a traditional combustion engine using petrol and hydrogen instead of hydrogen fuel cells, which Kammerer considers to be a niche car. ‘It is limited in power, size and ability for driving distance.’

This, according to Kammerer, makes them more versatile. ‘So if you are in an area where there are no hydrogen fuel stations, you can fill up with petrol,’ he said.

Kammerer said hydrogen is added to the tank through a manual tank coupling. Pure hydrogen is highly flammable, producing a great deal of energy when it reacts with oxygen, so extra precautions are taken to ensure the gas does not leak during refuelling.

In a liquid hydrogen-powered BMW, the hydrogen remaining in the tank has returned to a gaseous state by the time the driver needs more fuel. The gaseous hydrogen exerts a higher pressure inside the tank. At the refuelling station, when super-cold liquid hydrogen is pumped into the tank, the gaseous hydrogen already in there condenses. The condensation of the gaseous hydrogen reduces the pressure inside the tank, so no hydrogen escapes while the tank is being filled.

Kammerer said at the moment it takes up to eight minutes to fill up a liquid-hydrogen tank, but is hopeful this could be improved to four minutes in the future.